This invention relates to a system for rapidly raising the temperature of a product having low-thermal conductivity from a cold to a heated state in a manner bringing the internal temperature of the entire body of the product to the same elevated temperature level, and more particularly to a system of this type for use in conjunction with a convenience food service technique wherein a meal is first cooked, then refrigerated and stored and subsequently reheated without degrading the basic texture, flavor and nutritional qualities of the meal.
To meet the growing need for quickly-prepared, low-cost meals, convenience food operations have been developed in which the food to be served is first cooked, then deep-freezed and stored. When an order is placed for a particular item on the menu, the selected item is withdrawn from the freezer, the frozen pre-cooked meal is then thawed and reheated.
In reheating a pre-cooked frozen meal in homes and restaurants, it is difficult, when going from the frozen state to an adequately heated service condition, to avoid a situation in which the core of the product is still cold even though the outer layer is quite hot. And when one seeks to ensure that the body of the food is hot throughout, there is a tendency to overheat the meal and thereby re-cook it, with a resultant loss of nutritional value and flavor.
In my related U.S. Pat. No. 4,112,916, "Hot Air Oven for Food-Loaded Cartridges" (hereinafter referred to as the Oven patent), there is disclosed a fast food service technique and apparatus therefore whereby pre-cooked food which has been refrigerated may thereafter be reheated and made directly available to customers without degrading the essential texture, flavor or nutritional qualities of the meal.
The Oven patent discloses a hot air oven for heating tray-loaded cartridges, each constituted by a stack of sealed trays containing pre-cooked meals nested within an open carton whose side walls have holes therein to admit heated air. The oven includes a rotating turntable provided with a raised annular shelf for supporting an annular array of cartridges, the side walls of which define a hollow center core. A driven propeller is disposed within the core, the space between the shelf and the turntable forming a restricted flow passage whose inlet communicates with the core and whose outlet lies at the periphery of the turntable.
In this hot air oven, a heater assembly above the annular cartridge array produces heated air which is blown by the propeller into the hollow core. Because of the flow restriction, a substantial portion of the heated air is forced through the holes of the cartons to heat the food in the trays. The remaining portion of the heated air passes through the flow passage, the air discharged from the outlet thereof being drawn upwardly by the suction force of the propeller to create an air curtain around the cartridge array. Thus a toroidal flow pattern of heated air fully envelops the heated trays and serves to isolate the trays from the relatively cool ambient air without, however, interfering with direct access to the trays which may be withdrawn from the cartons when the food is at the desired temperature level for service to diners.
In an oven of the type disclosed in my Oven patent, a two-section heating assembly is provided having different wattages, whereby at the outset of heating, both sections are operative for a controllable period, hereinafter called the heat-up phase, sufficient to raise the food temperature to the desired service level, after which the main section is rendered inactive while the auxiliary section, which draws much less power, then serves to maintain indefinitely the heated food at the proper level for service to diners, hereinafter called the "service phase."
During the heat-up phase, the rate of heat transfer from the hot air in the oven to the relatively cold food-loaded cartridges depends on the temperature differential; the greater the difference between the hot air temperature and the food temperature, the more rapid the rate of heat transfer.
Since the hot air temperature throughout the oven is at a fairly uniform level, the transfer rate at the outset of heating in the heat-up phase when both heater sections are operative is very rapid, but as the difference in temperature between the hot air and the food thereafter diminishes, the rate of transfer becomes increasingly slow and quite sluggish as the service temperature is approached.
Assuming that the food in the cartridges is initially at a temperature of about 10.degree. F. and it is necessary to raise the food temperature to a service level of about 150.degree. F. and further assuming a hot air temperature of about 165.degree. F., then at the outset of the heat-up phase, there will be a sharp differential giving rise to very rapid heating. But as this temperature differential diminishes in the course of the heat-up phase, the rate of heat transfer slows down. When, for example, the food temperature reaches 130.degree. F., the temperature differential relative to the heated air is only 35.degree. F., and it takes a relatively long time before the food temperature can be raised to the service temperature of 150.degree. F., at which point the heat-up phase is concluded and the service phase takes over with only one heater section operative to maintain this service temperature level.
Thus if one plots a curve of cartridge food temperature (10.degree. F. to 150.degree. F.) vs. time in the heat-up phase, the resultant curve for a hot-air temperature of 165.degree. F. will exhibit a sharp rise from 10.degree. F. to about 100.degree. F. within a fairly short time interval, the curve thereafter leveling off as the temperature goes more gradually from 100.degree. F. to 150.degree. F. As a result, the duration of the heat-up phase is unduly prolonged, which in some situations may be a practical disadvantage.
If, for instance, a fast-food installation having a hot-air oven of the type disclosed in my Oven patent is loaded with cold food cartridges which must be made available for service to diners in about one hour after loading, this time may be inadequate to bring the food to its proper service level. If, therefore the cartridges containing the trays have just been removed from the freezer before being placed in the hot air oven, the necessary heat-up phase to raise the food temperature from, say, 10.degree. F. to 150.degree. F. with an oven of the type disclosed in this patent may be two hours or more, a period which is excessively long for some fast-food operations.
In order to provide an improved hot-air oven which affords a faster heat-up phase than an oven of the type disclosed in my Oven patent, my subsequent patent entitled "Two-Zone Hot Air Oven for Food-Loaded Cartridges" (hereinafter referred to as the Two-Zone patent) discloses an oven which functions to raise the food in the tray-loaded cartridges to a service temperature within a relatively short period.
In the Two-Zone patent, the hot-air oven is divided into a hot and an extra hot zone such that as the tray-loaded cartridges containing pre-cooked meals are carried by a turntable cyclically through these zones, the extra-hot zone acts to maintain a marked temperature differential between the temperature of the food trays and the hot air even as the food approaches the service temperature, whereby the transfer rate is rapid throughout the entire heat-up phase without, however, unduly heating the material of the trays.
In a hot-air oven for heating food-loaded cartridges of the type disclosed in my Two-Zone patent, each cartridge is formed by a stack of sealed trays all containing a pre-cooked meal, the stack being nested within an open carton whose side walls have holes therein to admit heated air. The oven includes a rotatable turntable provided with a slightly-raised annular shelf for supporting an annular array of cartridges forming a hollow center core whose boundary is defined by the walls of the cartons. Within the core is disposed a driven propeller, the space between the shelf and the turntable forming a restricted flow passage whose inlet communicates with the hollow core and whose outlet lies at the periphery of the turntable.
A heater assembly mounted above the cartridge array produces heated air which is blown by the propeller into the hollow core. Because of the flow restriction and the configuration of the carton walls, a substantial portion of the heated air is forced through the holes in carton walls in the cartridge array to heat the food in the trays, the remaining portion passing through the flow passage. The heated air escaping from the outlet of the flow passage is drawn upwardly by the suction force of the propeller to create an air curtain around the cartridge array which returns the air to the heater assembly for reheating and recirculation. The air curtain is surrounded by a thermal barrier having a front access port therein to provide access to the interior of the oven whereby cartridges may be inserted in the oven when loading the shelf and selected trays may be withdrawn therefrom when dispensing meals to diners.
The heater assembly is constituted by two concentric arcuate sections of different capacity, both of which are initially energized to raise the oven temperature to a high level and to bring the food to the desired service temperature in the course of a heat-up phase, after which the larger capacity main section is cut off automatically by means of a timer, whereby the food in the oven is thereafter maintained at the desired level during a service phase by the smaller auxiliary section.
Thus a toroidal flow pattern of heated air is created which fully envelops the annular array of cartridges and serves to isolate the trays from the relatively cool ambient air, without, however, interfering with direct access to the trays which may be withdrawn from the cartons through the access port when the meals are at the desired service temperature.
In order to accelerate the rate in the heat-up phase at which the food is raised to its service temperature, disposed within the hollow core of the oven in the sector thereof facing the access port is an arcuate shield which acts to restrict the passage of heated air through the holes of cartons in the shielded sector of the core so that as the turntable rotates, the cartridges are subjected to heated air from the core only when they travel through the unshielded sector.
As a result, the oven is effectively divided into a shielded hot zone and unshielded extra-hot zone, the heater arrangement being such that the temperature in the extra-hot zone is well above the service food temperature and is even above the softening point of the plastic trays. But because the turntable during each cycle of rotation at 1 RPM carries the cartridges from the extra-hot zone to the hot zone whose temperature is below the softening point of the trays, the trays never reach their softening point in the course of the relatively short heat-up phase.
The two-zone oven accelerates the food heating process; for even when the food temperature approaches the service temperature, there is still a marked temperature differential between the food temperature and the super-hot temperature to promote more rapid heat transfer. Thus instead of a time-temperature curve in the heat-up phase which rises steeply and then gradually levels off, the curve remains relatively steep throughout the entire heat-up phase, thereby shortening the duration of the heat-up phase.
Though the technique disclosed in my Two-Zone patent is adapted to rapidly heat-up pre-cooked food to a service temperature and to thereafter maintain the food at this temperature, the generally-cylindrical oven structure which includes a turntable for carrying out this technique is designed to handle a large volume of food-loaded cartridges. This structure can be scaled down to create a small unit suitable for home use operating on the same principles. But a cylindrical unit, in many instances, does not lend itself to a home or other installations having space restrictions.
In a typical domestic kitchen, a box-like unit for reheating frozen pre-cooked food packages is more appropriate. Box-like home units operating a convection heating principles are known and are commercially available. However, such units are incapable of rapidly heating up trays or containers of pre-cooked food and thereafter maintaining the food at a desired service temperature.
In convection-heating home units of the type heretofore available, one can set the oven for a desired heat-up temperature. But as previously explained, the temperature differential between the cold food and the heated air is large only in the initial heat-up period, and the closer the food approaches the service temperature, the smaller the differential and the slower the rate of heat transfer. Consequently, it takes an unacceptably long time for the food to reach the service temperature. This is particularly the case when the unit is fully loaded with several trays or packages of frozen food.
If the operator of the typical home unit tries to accelerate the heat-up phase by setting the temperature level of the oven well above the service temperature, the resultant heating will generally be destructive of the food; for the outer layers of the food will then be heated to an excessive level, causing these layers to be re-cooked or burned.
Again, it must be borne in mind that frozen food has poor thermal conductivity; hence the rate of heat transfer within the body of the food is slow. If, therefore, one subjects a body of frozen food to a continuously maintained high temperature, the outer layers of the body will undergo rapid heat transfer and become overheated before significant heating takes place in the inner regions of the body.